SUMMARY Partition coefficients were determined for halothane in various rabbit tissues after about five hours anaesthesia. After correcting for the fall in haematocrit which occurred during the experiments the mean blood/gas coefficient was 3.94. Except in the case of skeletal muscle the tissue/blood coefficients differed significantly from unity: brain 1.55, heart 2.74, kidney 1.75 and liver 2.26. The differences between the tissues were highly significant. Differences between animals were large but mainly unsystematic. The 95% confidence limits for tissue/gas coefficients in the population were +32% —24%. There was also variation between individual 1- to 2-ml samples of the same tissue: the 95% confidence limits for such samples were of the order of +20% of the mean for liver, and +50% of the mean for heart, kidney and muscle.